1. FIELD OF THE INVENTION
The invention relates to a non-ageing low-alloy hot-rolled strip-form formable steel. The steel has good mechanical and surface properties. Typically such a steel has a thickness in the range 0.5 to 5.0 mm.
2. DESCRIPTION OF THE PRIOR ART
A low-alloy hot-rolled steel strip of a given thickness and a given C-content may be obtained in accordance with a known method in which a cast steel slab with a thickness of between 25 and 300 mm is first cooled down and then before hot-rolling heated up to, and homogenized at, a temperature of between 1100.degree. C. and 1250.degree. C.
As the slab cools down any nitrogen present in the steel combines with aluminum into aluminium nitride. This binding of N is preserved as the slab is heated up again if the temperature is kept not higher than about 1100.degree. C. This gives a product which is non-ageing.
If the steel is heated to higher temperatures, free nitrogen goes increasingly into solution in the steel, which means that the steel becomes less non-ageing. This ageing occurs in particular after pickling and re-rolling of the hot-rolled steel strip. This ageing has a disadvantageous effect on the mechanical properties of the formable steel. During forming of age-hardened steel, flow lines develop on the surface, which means that the surface quality of the finished product is not ideal.
Also if the slab is not sufficiently cooled before being re-heated, or if it is taken directly from the casting heat into an homogenizing furnace, free nitrogen will remain present in the steel, because nitrides are unable to form. Then also non-ageing material is not obtained, even if the furnace temperature is low. For reasons of energy saving and stock limiting it is becoming increasingly common not to cool the cast slabs down to ambient temperature but to place them in the furnace at a higher temperature or even send them through a furnace directly after casting. In addition, for reasons of savings in energy and savings in material and product properties, it is sought to reduce the furnace temperature down to 1000.degree. C. or even down to 850.degree. C. So, in this known method, the energy saving desired is impossible if it is intended to manufacture a non-ageing product.
It is to be noted that dissolved nitrogen in the steel may also have a disadvantageous influence on the achievement of a good strip shape and an even thickness. In particular this is the case when, for reasons of energy saving, a low temperature in the re-heating furnace is sought. A low temperature in the re-heating furnace implies low rolling temperatures. At these relatively low rolling temperatures, dissolved nitorgen impedes a complete recrystallization of the steel between the different forming stages in the hot-rolling process. This means that the hardness of the steel may vary considerably during forming, leading to the drawbacks described.
Dissolved nitrogen may also prevent a complete recrystallization of the hot-rolled steel, if the steel is coiled at a temperature below 700.degree. C. A coiling temperature below 700.degree. C. is desirable from the point of view of oxide control and homogeneity of mechanical properties. The level of mechanical properties is seriously affected by incomplete recrystallization.
Some specific prior art proposals will be referred to below, where they are more easily discussed in the light of the following explanation of the present invention.